It is desirable to provide a Near Instantaneous Companded Audio Multiplex (NICAM) system that it can be integrated into a large SOC (system on a chip) efficiently. The method of this invention allows the demodulation and decoding of NICAM without requiring any phase locked loops (PLLs) or feedback. This invention can be built using only a standard A/D converter, a digital signal processor (DSP) core and logic gates. Because this method does not include a PLL or additional analog demodulation circuitry, it permits efficient implementation in an advanced digital process.
At the receiver, the tuner converts the video carrier and the F.M. sound inter-carrier to respective intermediate frequencies (IF) of 39.5 MHz and 33.5 MHz in the normal way. The NICAM carrier (which is 6.552 MHz away from the video carrier) is converted to an intermediate frequency of NICAM IF of 39.5 MHz-6.552 MHz=32.948 MHz or approximately 32.95 MHz.
This IF signal is demodulated by a digital quadrature phase shift keying (DQPSK) detector and applied to the NICAM decoder which reverses the transmitter encoding to recreate the 14-bit sample code words for each channel. A digital-to-analog converter reproduces the original analog two-channel, left and right sound waveforms.
FIG. 1 illustrates the basic elements of NICAM sound reception in a TV receiver. Antenna 110 receives radio frequency signals and supplies them to tuner 120. Tuner 120 selects the desired radio frequency signal and supplies an intermediate frequency (IF) signal to special surface acoustic wave (SAW) filter 130. SAW filter 130 separates the video and sound IF outputs. A sharp cut-off removes the two sound IFs, 33.5 MHz for mono and 32.95 MHz for NICAM, from the video 39.5 MHz carrier. SAW filter 130 generates separate outputs for the video and NICAM IF carrier. SAW filter 130 also provides for a very narrow peak at 39.5 MHz. Sound. IF demodulator 140 uses the 39.5 MHz pilot frequency to beat with the FM sound IF signal to produce 6 MHz FM mono audio signal and with the NICAM IF to produce 6.552 MHz DQPSK carriers. Sound IF demodulator 140 uses sharply tuned filters to separate the two sound carriers. The FM carrier goes to a conventional FM processing channel 155 for mono sound. The 6.552 MHz NICAM phase modulated carrier goes to a NICAM processing section. This includes three basic parts. DQPSK decoder 160 recovers the 728 kbit per second serial data stream from the 6.552 MHz carrier. NICAM decoder 170 de-scrambles, de-interleaves, corrects and expands the data stream back into 14-bit sample code words. Finally, digital-to-analog converter 180 reproduces the original analog signals for each channel.
DQPSK demodulator 160 works on the same principle as a frequency modulation (FM) detector. A variation in phase or frequency produces a variation in the direct current (DC) output. In the case of two-phase modulation, the DC output of the detector has two distinct values representing logic 1 and logic 0. However, in the case of quadrature, i.e. four-phase modulation, the output of the detector is ambiguous. The same output for a 90° phase shift is obtained as that for a phase shift of 270°. This is similar for phase shifts of 0° and 180°. In order to resolve the ambiguities, a second phase detector operating in quadrature (90°) is typically used.
FIG. 2 illustrates the main elements of a DQPSK demodulator 160 of the prior art. The input NICAM 23.95 MHz IF signal in input to band pass filter 150. The output of band pass filter 150 supplies the inputs of both in-phase phase detector (PDI) 210 and quadrature phase detector (PDQ) 220. The output of in-phase phase detector 210 supplies the input of low pass filter 215. The output of quadrature phase detector 220 supplies the input of low pass filter 225. The filtered outputs from the two phase detectors are the data I and data Q. These feed data recovery circuit 170 which reproduces the original serial data stream. In the NICAM standard this is a 728-bit serial bit stream. Carrier recovery block 230 recovers the 6.552 MHz reference carrier frequency from the in-phase filtered signal. Carrier recovery block 230 supplies this recovered carrier to in-phase phase detector 210 to beat with the input. Carrier recovery block 210 supplies 90° phase shifter 235 which supplies this phase shifted signal as the beat carrier to quadrature phase detector 220.
FIG. 3 illustrates an example of the quadrature encoding of the NICAM standard. Two bits of data are encoded in a phase shift of the carrier. A 0° phase shift encodes the binary pair “00.” A 90° phase shift encodes the binary pair “01.” A 180° phase shift encodes the binary pair “10.” Finally, a 270° phase shift encodes the binary pair “11.” The decoding task is to unambiguously determine the transmitted phase shift to recover the encoded bit pair.
This prior art technique has disadvantages making construction of low-cost systems difficult. A DQPSK demodulator such as illustrated in FIG. 2 typically requires a phase locked loop (PLL) in carrier recovery. In current technology it is difficult to construct a single integrated circuit including both the analog components needed for such as PLL and digital data processing circuits. A typical TV requiring NICAM demodulation will include a high-performance digital signal processor (DSP) for many image and audio tasks. Because of the difficulty of constructing a single integrated circuit including both a PLL and a DSP, more circuits are needed for the TV system. This results in increased cost.